Process for the continuous feeding of electrolytic aluminum cells



July 12, 1955 G. MANTOVANELLO 2,713,024

PROCESS FOR THE CONTINUOUS FEEDING OF ELECTROLYTIC ALUMINUM CELLS Filed May 10, 1951 2 Sheets-Sheet l Ri ES M:

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INVENToR ANNY/m5 July 12, 1955 G. MANTOVANELLO 2,713,024

PROCESS FOR THE CONTINUOUS FEEDING OF ELECTROLYTIC ALUMINUM CELLS Filed May 10, 1951 2 Sheets-Sheet 2 Fig. 5

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United States Patent PROCESS FGR THE CGNTINUOUS FEEDING OF .ELECTRGLYTIC ALUMINUM CELLS Giovanni lvlantovanelio, Milan, Italy, assignor to Montecatini, soc. gen. per IIndustria Mineraria e Chimlca, a corporation of Italy Application May 10, 1951, Serial No. 225,482

Claims priority, application Italy June 13, 1950 Claims. (Ci. 20467) It is known that in operating cells for the production of aluminum by the electrolytic reduction of alumina in a bath of fused cryolite or fluorides in general, the feeding of alumina is carried out in a discontinuous manner at the time at which the anodic effect occurs. Ordinarily the feeding is carried out by breaking the frozen top crust of the bath with a tool and introducing a predetermined charge of alumina previously loaded onto said crust.

The anodic efiect occurs in electrolytic cells when the concentration of alumina dissolved in the fused bath v drops below a certain critical limit; so that the resistance to the passage of electric current increases and the voltage of the cell, which in normal operation is generally between 4 and volts, rises to values of about to volts.

With such a discontinuous feeding operation the concentration of aluminum oxide in the fused electrolytic bath varies continuously between a maximum and a minimum, from the instant when the alumina is introduced immediately after occurrence of the anodic effect, to the instant, when the anodic effect re-appears. This causes higher consumption of energy and smaller current efficiency than would be attainable if the composition of the bath could be kept constant by introducing the alumina into the bath continuously.

The attempt was made in the past to feed the alumina into the cell at a time prior to the occurrence of the anodic efiect, taking into account the time necessary for the electrolysis of the alumina solution. This method is unsatisfactory because at the feeding moment the previous charge of alumina may not be fully dissolved, and thus the anodic effect may happen earlier than foreseen; and also because the charge of alumina introduced out of time may not dissolve because it exceeds the solubility of alumina in the fused bath at the working ten perature and hence may precipicate to the bottom of the cell thus causing irregular operation.

The best way of overcoming these deficiencies is to introduce the alumina into the fused bath continuously a quantity corresponding to that being electrolyzed in the cell, so as to maintain a constant percentage of dissolved alumina. However, the methods so far proposed have failed to produce a useful result because they involved simply dropping the alumina onto the surface of the bath. Even if such gravity feeding is performed at the hottest or heat-insulated points of the cell, the entrance of alumina, pre-heated or not, is opposed by the formation of a top crust on the bath so that intervention from the outside is necessary. At best, this method can provide a semi-continuous feed but remains affected by the above-described disadvantages.

It is an object of the present invention to secure a continuous and regular operation in a simple manner. To this end, and in accordance with the invention, the alumina is fed into the bath under a continuous pressure sufiicient to overcome the mechanical resistance of the superficial crust so that the alumina is forced to penetrate into the molten bath portion.

The pressure for the continuous andregular penetration of alumina into the fused bath is applied, according to another feature of the invention, by mechanical means, preferably by a screw-type thrust feeder, or by compressed air or gas.

The introduction of the alumina into the bath according to this method may be effected at any point of the cell; at any rate, the dissolving of the alumina is facilitated if the introduction takes place in the proximity of the electrodes, where the agitation in the bath is a maximum due to convection currents or due to the evolution of anodic gases.

The process may be applied to any type or form of cells for instance, with single or multiple pre-baked electrodes, or with Siiderberg type self-baking electrodes in single or multiple arrangements. Moreover, the process is not limited to cells of any particular output capacity, it being obvious that the number of units for the pressure feeding of alumina may be varied at will according to the various requirements.

In the accompanying drawings, the invention is illustrated by way of example with reference to embodiments equipped with screw-type thrust feeders.

Fig. 1 represents diagrammatically a device for the application of the process with a central screw-type feeder mounted within a continuous soderberg electrode.

Fig. 2 represents diagrammatically a device with two screw-type thrust feeders within a Siiderberg electrode.

Fig. 3 represents diagrammatically a device with a screw-type thrust feeder outside a Socierberg electrode.

Fig. 4 represents diagrammatically the application of the process with a screw-type thrust feeder in the case of a multiple anode furnace.

Fig. 5 represents diagrammatically the application of the process with a screw device acting as a thrust feeder as well as a measuring device.

Fig. 6 represents diagrammatically the application of the process with compressed air or gas.

Fig. 7 represents diagrammatically the application of the process with a piston system.

The same reference numerals are used in the various illustrations to denote functionally similar elements respectively.

The electric furnace cell shown in Fig. 1 has the cavity of its lined vessel 13 filled with a quantity of a fused fluoride bath 4. Partially submerged in the bath is an electrode 11. The bath is supplied with alumina from a tank or feed hopper 1. The alumina is discharged from the hopper by means of a screw-type measuring device 2 into a pipe 3 and is injected into the bath 4 by means of a screw-type thrust feeder 5. The assembly is actuated by an electric motor 6, which drives the measuring device 2 and the screw 5 through a coupling 9 and through respective gear boxes '7 and 8. A metallic feed pipe 19 containing the screw 5 extends down to a height in the order of some centimeters above the surface of the bath 4, taking into account the oscillations in bath level. Such a spacing is sufficient to prevent contamination of the bath and is also small enough for having the screw (or equivalent pressure means) impose the necessary thrust upon the bath crust. To make allowance for any rise in bath level due to the rise of the cell bottom, the pipe 10 containing the screw 5 may be made adjustable in height with any suitable means (not shown). The bore in electrode 11, wherein the pipe 10 is accommodated, is determined by a circular liner 12 fixed to the furnace roof or other structure (not shown).

Of course, a plurality of feeding units may be actuated, by the same motor or by separate motors as represented Patented July 12, 1955,

3 in Fig. 2, to eifect feeding at furnace.

The feeding may be performed externally of the electrode, that is between the electrode and the side wall of the furnace, as illustrated in Fig. 31 In this case, the descending column of alumina powder is still guided by the feed pipe 19 containing the screw 5, but is no longer guided by an electrode boreJ However, as has been found, the alumina does not tend to spread and clog outsidethe feed pipe. Due to the feed pressure applied by the screw in the illustrated embodiment, the alumina below the pipe forms a truncated cone having its base on the fused bath or crust. Under the effect of. the feed pressure the base of the cone grows only until it reaches a certain size, and the continuing pressure, acting in the vertical direction, then breaks the crust or prevents it from forming.

The feeding device may be mounted on any type furnace, for instance also on furnaces with multiple anodes 14 as illustrated in Fig. 4.

In the embodiment represented in Fig. 5, the alumina passes from the feed hopper 1 through a pipe 15 directly into the feed pipe 10, and the screw 16 contained in pipe 10 acts at the same time as a thrust-screw and as a measuring device. The screw 16 is driven by the motor 6 through a gear box 17.

In the embodiment of Fig. 6, the alumina passes from feed hopper 1 through a metering and dispensing screw 2 and a connecting duct 3 into a feed pipe 18 that traverses a number of points of the a bore of the electrode 11 but does not contain a pressure screw. Instead, the continuous feed pressure is applied by means of compressed air. The compressed air passes through a regulating valve 20 into an inlet conduit 19 which terminates in a nozzle 21 within pipe 18 at a point above the alumina inlet opening.

within the scope of my invention any mechanical system may be used embodying the continuous introduction of alumina into the fused bath of an electrolytic aluminum cell under an external pressure overpowering the resistance of the bath against the entrance of the oxide. It has been ascertained in practice that the process according to the invention effectively reduces the consumption of energy and also improves current efiiciency, as the anodic effect is virtually suppressed. The substantial suppression of the anodic effect (for 24-48 hours or more) has the further advantage that the furnace can be run at constant power, that is with automatic voltage regulation.

Moreover it has been found that under these conditions the thermal balance of the cells is more favorable and that a smaller consumption of correctives for the electrolysis bath and, of course, less labor for running the cell are required. It has been further ascertained that the purity of the metal can be improved because the invention prevents the occurrence of contaminations that, with a discontinuous feed, may enter into the bath due to the use of the metallic tools necessary for breaking the crust.

I claim:

1. In the production of metallic aluminum by the electrolytic reduction of alumina in a bath of fused electrolyte, the process which comprises maintaining the bath at crust-forming temperature conditions, continuously feeding alumina through the crust into the bath, and applying feed pressure through the alumina to the bath surface to thereby penetrate the crust.

2. In the production of metallic aluminum by the electrolytic reduction of alumina in a bath of fused electrolyte, the process which comprises maintaining the bath at crust-forming temperature conditions, feeding alumina under pressure through the crust into the bath, and maintaining a continuous rate of feed to keep the bath surface free from crust at the entrance-place of the alumina.

3. in the production of metallic aluminum by the electrolytic reduction of alumina in a bath of fused electrolyte, the process which comprises maintaining the bath at crust-forming temperature conditions, feeding alumina into the bath through a substantially vertical pipe terminating closely above the surface of the bath, and applying pressure to the alumina withinthe pipe to force the alumina through the crust.

4. In the production of metallic aluminum by the electrolytic reduction of alumina in a bath of fused electrolyte, the process which comprises maintaining the 'bath at crust-forming temperature conditions, feeding a continuous column of alumina from above through' the crust into the bath at a place in closely adjacent to the outside of the electrolysis electrode and applying to the column a substantially continuous feed pressure to thereby penetrate the crust. p

5. in the production of metallic aluminum by the electrolytic reduction of alumina in a bath of. fused electrolyte, the. process which comprises maintaining the bath at crust-forming temperature conditions, feeding a continuous column of alumina through a bath electrode into the bath, and imparting through the column of alumina a feed pressure upon the alumina-em trance place of the bath surface to thereby penetrate the crust.

References Cited in'the file of this patent France Feb. 28; 1928 

1. IN THE PRODUCTION OF METALLIC ALUMINUM BY THE ELECTROLYTIC REDUCTION OF ALUMINA IN A BATH OF FUSED ELECTROLYTE, THE PROCESS WHICH COMPRISES MAINTAINING THE BATH AT CRUST-FORMING TEMPERATURE CONDITIONS, CONTINUOUSLY FEEDING ALUMINA THROUGH THE CRUST INTO THE BATH, AND APPLYING FEED PRESSURE THROUGH THE ALUMINA TO THE BATH SURFACE TO THEREBY PENETRATE THE CRUST. 